Conventionally, there has been known a mobile communication system capable of accommodating radio access systems in the 2G/3G schemes (UTRAN: Universal Terrestrial Radio Access Network/GERAN: GSM EDGE Radio Access Network) and a radio access system (E-UTRAN: Evolved Universal Terrestrial Radio Access Network) in an LTE (Long Term Evolution) scheme.
In such mobile communication systems, there has been known a technology that switches a UE (User Equipment, a mobile station) #1 from a state of performing voice communications using a voice communication packet bearer in a PS (Packet Switch, packet switched) domain (state of performing VoIP communications using an INS (IP Multimedia Subsystem, service control network)) to a state of performing voice communications using a circuit-switched bearer in a CS (Circuit Switch, circuit switched) domain, so that the voice communications of the UE#1 can be continued, i.e., “SRVCC (Single Radio Voice Call Continuity)” (see, Non-patent document 1). FIG. 2 shows an operation of switching the voice communications in the mobile communication system.